Preheat oven to 350°; wash sweet potatos well and dry with a clean cloth; slice each potato in half lengthwise.
Oil a glass baking dish or a standard baking sheet with sides.
Pour 1 tablespoon oil into a small bowl (pour less oil than you think you'll need and add more if you think you run low).
Dip your fingers into the oil and rub oil over each sweet potato half, both the cut surface and the skin.
When each potato half is coated, place it in the dish cut side up.
Coarsely chop the rosemary and sprinkle it over the potatoes; sprinkle with salt and pepper to taste.
Slide pan into oven and roast the potatoes for 40-50 minutes.
When the potatoes are just about ready, roll the orange between your palm and the countertop to get the juices flowing, then juice the orange and set the juice aside (if you like, you can zest the orange before juicing and toss a tiny bit of zest into the juice).
As soon as the potatoes come out of the oven, score the cut surface of each potato using a sharp knife, making a slight checkerboard pattern; spoon a little juice over the top of each potato; don't drown them, just give them a bright hit of citrus to balance the rosemary, salt, and pepper.